Such cutting inserts with integrally formed screw threaded coupling shanks have long been known and form the subject, for example, of British Standard 122-Part 4. Such a cutting insert is also disclosed in French Patent Specification No. 2602162.
Replaceable cutting inserts in general, and especially the cutting heads thereof, are usually formed of compacted sintered hard metals such as, for example, tungsten carbide with a suitable binder. The provision of such hard metal cutting heads with integrally formed screw threaded coupling shanks has not been hitherto proposed. It is clear that there are inherent difficulties in forming screw threading on a coupling shank formed by powder metallurgy techniques. Thus, any attempt to provide the coupling shank during a compacting process with the required screw threading, would invariably result in the production of a ridge of excess material in the region where two die punches approach each other. This is in view of the fact that the adjacent edges of the die punches have to have a minimal thickness in order to avoid punch breakage. This ridge, must clearly be removed by subsequent machining in order to allow for screw coupling.
It has been proposed in U.S. Pat. No. 5,248,474 to manufacture by hot isostatic pressing and sintering of metal powder, large diameter metal parts for use as electrodes for high temperature electric arc furnaces. Such metal parts are, after sintering, subjected to machining so as to form thereon suitable threading. Clearly, applying this concept of subsequent machining of an insert formed by a powdered metallurgy technique so as to impart to it appropriate threading would increase the costs of manufacture of the insert.
It is an object of the present invention to provide a new and improved cutting insert having an integral screw threaded coupling shank in which the above-referred-to difficulties are substantially reduced or overcome.